I had a similar question earlier today.
In the thread: "Shock, Despair, then Redemption," Ginseng explained why he prefers the 2 x AA to the 123:
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Ginseng said:
Use
1. It fits the hand better during use. The clickie is perfectly positioned for thumbing while maintaining a full four-finger grip. The 123 clickie pack is very hard to hold and activate without applying a deathgrip and putting your thumb in an awkward position.
Carry
1. It fits in my RipOffs CO-80 holster in which I also carry my Leatherman Wave. The Arc belt holster that fits the 123 pack frankly sucks. If you prefer to keep it in your pocket, then the shorter pack is preferable. But it is too fat for my pockets. While the 123 pack has a split ring hole, I find it really too heavy as it keeps banging into my hip.
Output
1. You get 3:15 or more of max runtime with 2XAA lithiums versus 2:15 with the single CR123. And the light is measureably, though probably not noticeably, brighter with the 2AA pack.
How's that?
Wilkey
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I hope his insight is useful to you. It was to me.